i8 MOVEMENT 



some evidence as to the expansion or contraction in the antagonistic tissues. Thus 

 the rigidity will increase if the force of expansion becomes greater either in one or 

 in both halves of the pulvinus, but will be lessened if it falls in one or both halves, 

 while if the rigidity remains constant we have evidence to show that one side expands 

 and the other contracts in equal degree. Briicke l measured the rigidity by noting 

 the bending when the organ was held horizontally, firstly with the curvature upwards, 

 then with it facing downwards. The angular divergence was read off on an arc 

 having its centre at the median point of the pulvinus. In the same way an increased 

 rigidity is shown when a load produces less bending in an organ kept in the same 

 horizontal position 2 . 



During its geotropic curvature a root may lift a considerable weight, and by 

 finding the weight required to prevent curvature a measure may be obtained of the 

 energy of curvature 3 . Slender plastic roots which are easily bent can naturally 

 exert no great pressure unless lateral displacement is prevented. Nutation move- 

 ments may also take place against considerable external resistance since they result 

 from irregular growth. In such cases the external resistance antagonizes a portion 

 of the osmotic pressure acting against the stretched cell-wall. Similarly in variation 

 movements, either a rise of turgor takes place or a fall enables the previous stretching 

 of the cell-wall to come into play. 



If the antagonistic tissues are symmetrically displaced, as in a radial organ, no 

 curvature is shown until the organ is split in half. The energy of curvature is 

 greater when the active tissues are some distance from the neutral axis, since the 

 leverage or bending moment they exert is increased. The bending moment 

 therefore depends upon the energy of expansion and upon the distribution of the 

 active tissues 4 . The problem is the same whether the curving zone is short or long, 

 and the curving zone may in fact be made extremely short by preventing the 

 attempted movement by means of bandages over the greater part of the length 8 . 



Thin organs can naturally bend more sharply and rapidly than thick ones, since 

 in the latter a considerable difference in length has to be produced between the 

 convex and concave sides 6 . The most pronounced curvature does not always 

 occur in the most actively growing zone, since the conditions for curvature are often 

 later in development. Curvature is influenced by external conditions in exactly the 

 same way as is growth in general. In certain cases, as in tendrils, it is favoured by 

 abundant supplies of water, whereas the movement of the pulvini of Mimosa is for 

 obvious reasons decreased or prevented when the intercellular spaces are injected 

 with water. 



1 Briicke, Miiller's Archiv f. Physiol., 1848, p. 452. Cf. Pfeffer, Period. Bewegungen, 1875, p. 89. 



2 Schwendener (1897), Gesammelte Abhandlg., Bd. n, p. 237. 



3 The best form of apparatus is a very stiff spring which can be adjusted by a screw. Cf. Pfeffer, 

 Period. Bewegungen, 1875, pp. 9, 97 ; Druck- und Arbeitsleistungen, 1893, p. 389 ; Meischke, Jahrb. f. 

 wiss. Bot., 1899, Bd. xxxin, p. 345. 



* Cf. Pfeffer, 1875, 1. c., p. 99 ; 1893, 1. c., p. 392. 



5 Meischke, 1. c. , p. 348. 



6 Cf. Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. vil, p. 173. 



